The present invention relates to a coil assembly including tubes made of synthetic polymeric material, rather than metal, such as galvanized steel or stainless steel, referred to hereinafter generally as a “polymeric coil assembly,” for use in heat exchange apparatus, particularly evaporative heat exchange apparatus, and to a method of making the polymeric coil assembly. Other aspects of the invention are set forth hereinafter.
The polymeric coil assembly may be used by manufacturers in original heat exchange equipment, typically but not exclusively cooling towers, or as a replacement coil assembly in previously installed heat exchangers. It is particularly useful as a replacement coil assembly in existing installations, since the weight of the polymeric coil assembly is considerably less than the weight of a typical coil assembly made of galvanized steel or stainless steel. For ease of explanation, reference will be made to cooling towers, although the polymeric coil assembly may be used with any other heat exchange equipment where coil assemblies are used for indirect heat exchange between a fluid flowing internally through the tubes of the coil assembly and fluid flowing externally over the tubes in the coil assembly. The coil assemblies of the present invention may be used in systems that operate in wet and dry cooling operations and in closed loop and open loop operations. The polymeric coil assembly may be used in indirect heat exchange systems or together with direct heat transfer media, such as contact bodies that may be splash bars, corrugated synthetic polymer sheets or any other type of direct heat transfer media.
In evaporative heat exchangers using indirect heat transfer provided by coil assemblies, the coil assembly is mounted in a duct or housing in the apparatus in which heat exchange fluids, typically a liquid, usually water, and a gas, usually air, flow externally through the coil assembly to cool or condense a heat transfer fluid passing internally through the tubes of the coil assembly. Coil assemblies, including the polymeric coil assembly of the present invention, may be mounted in a counterflow evaporative heat exchanger so that water flows downwardly and externally through the coil assembly while air travels upwardly and externally through the coil assembly. Coil assemblies, including the polymeric coil assembly of the present invention, also may be mounted in a cross-flow evaporative heat exchanger where the external liquid, usually water, flows downwardly and externally through the coil assembly while the gas, typically air, travels generally transversely across and externally through the coil assembly before reaching a plenum where the air typically exits the top of the apparatus. It is also possible to mount the polymeric coil assembly of the present invention in an evaporative heat exchanger where the external liquid, usually water, and the gas, typically air, flows concurrently or in parallel downwardly and externally through the coil assembly. In any of these arrangements, the evaporation of the water on the external surface of the tubes of the coil cools the coil assembly by conduction and the internal heat transfer fluid inside the tubes by convection.
The water is sprayed by water distribution pipes and nozzles or from troughs where the water flows by gravity over the coil assembly. The water is typically collected in a sump at the bottom of the heat exchanger and recirculated often with makeup water added due to the evaporation to the water distribution system by appropriate pumps through valves in a well-known manner. The air may be inductively drawn into the apparatus and externally through the coil assembly by exhaust fans that are typically propeller fans and then exhausted typically out of the top of the heat exchanger, or the air may be propelled externally through the coil assembly in a forced draft manner by various types of fans, such as centrifugal blowers.
Coil assemblies typically are made from metal tubes that run across the cooling chamber or housing of the evaporative heat exchangers, where the tubes are most often arranged as a series of serpentine tubes including generally straight portions, sometimes called segments, are connected together by bends in the tubing, sometimes referred to as “bights” or as in this application “return bends.” The serpentine tubes of the coil assemblies may be packed tightly within the heat exchanger section of the cooling tower with the segments on staggered vertical levels so that the water and air can travel externally through the coil assembly, or the serpentine tubes can be arranged to have some horizontal spacing between the segments on adjacent vertical layers of tubes. There is a trade-off between the power requirement to draw or force air externally through the coil assembly usually against or across the force of water in counterflow and cross-flow arrangements, and the surface area of the tubes in the coil assembly available for heat transfer, which is impacted by the size, number and spacing of the tubes in a coil assembly.
Cooling towers are the most efficient means to reject heat from warm process water used in many applications such as power generation, food processing and HVAC. Closed loop fluid coolers can reject heat from the warm process liquid flowing internally through the tubes indirectly to the outside air stream by transferring heat through the tube walls. Usually fluid cooler heat exchangers are made out of galvanized steel for corrosion purposes from constant exposure to the water spray. However, the galvanizing coating is only on the outside of the tubes. Therefore, circulating water from an open system which is continually aerated will quickly deteriorate the inside of the steel tubing. For this reason, much more expensive stainless steel tubing coils are required for fluid coolers operating in an open system.
Polymeric coil assemblies have been proposed before and have found some success in the market, such as polymeric coil assemblies made and sold by PowerCold Corporation. The polymeric coils of PowerCold Corporation included a serpentine coil assembly made by folding an elongated bundle of polymeric tubes, but there were issues regarding the heat transfer through the bundle, especially at the return bends. The PowerCold polymeric coil assemblies had tubes that were not spaced regularly in each bundled straight section or “pass,” and often were laid against each other in sections between spacers transverse to the longitudinal direction of the pass. The tubes were threaded through holes in the spacers using a steel rod to support the tubes as they were threaded through the holes. The spacers snapped together to form a complete coil row. Additionally, a rod was inserted through the assembled spacers to attach and support an external frame structure. The tube bundle was split into two groups of thirty tubes per bundle per pass. At each return bend, the top bundle of tubes in the pass switched with the lower bundle of tubes in the pass to minimize the tube bend and prevent kinking. The return bends were bundled, taped or twisted together, in effect preventing or severely restricting the external flow of air and water through the return bends and losing significant heat transfer capacity at the return bends. The bundles of the tubes transitioned at their ends from a square or rectangular spacing pattern into a round tube pattern, since the ends of the tubes were sonically welded into a round tube sheet to fit into a standard polyvinyl chloride (PVC) pipe union. The polymeric coil assembly of the present invention overcomes the problems associated with the PowerCold polymeric tube assembly.
The polymeric coil assembly of the present invention provides a maximum amount of coil heat transfer surface area within a given coil assembly volume, enhancing heat transfer capacity, with significantly less weight than a metal coil assembly of the same dimensions. The polymeric tubing used in the polymeric coil assembly is less prone to corrosion than galvanized steel coil assemblies. Compared to prior art polymeric coil assemblies, the polymeric coil assembly of the present invention avoids bundling of the tubes in the passes and bundling, taping or twisting of the tubes at the bends, but instead provides appropriate spacing of the tubes such that they minimize sagging and maintain substantially the same relative positional relationship to each other in the passes and in the return bends. This enhances heat transfer capacity and efficacy.
A method of making the polymeric coil assembly is part of the invention. The method assures that the tubes in the array maintain substantially the same relative positional relationship to each other in the passes and in the return bends.
Another part of the invention concerns making any type of coil assembly of polymeric tubes and forming tube sheets around the ends of the tubes by injection molding, casting or potting. This method is more efficient than threading the polymeric tube ends through alignment holes in the tube sheets that connect the tubes to manifolds for heat transfer fluid to flow internally through the tubes in the coil assembly.
Similarly, the invention also includes a method for forming spacers around polymeric tubes in any type of coil assembly by a casting or potting the spacers, of which there are usually many, around the tubes, again avoiding the need to thread the tubes through many spacers.
Cooling towers are known that separately include both coil assemblies and direct contact evaporative heat exchange fill media in the heat exchanger housing. Another part of the present invention is to combine direct contact evaporative heat exchange fill media into a coil assembly for purposes of at least one of enhancing heat exchange characteristics, water redistribution over at least one section of tubular coils and to support adjacent passes of the coils.
Evaporative cooling towers also use water distribution systems that typically include nozzles to spray water upon coil assemblies and/or direct contact air/water evaporative fill media. The water distributors are separate parts of the heat exchange apparatus. Another aspect of this invention is the integration of a water distribution subassembly within a coil assembly of polymeric tubes of any construction. By incorporating a water distribution subassembly integrally in a coil assembly the water can be distributed precisely where it is intended for a particular application.